It looks like the Saturday game is supply level, so this will be more formal, including ARs. It’s nothing to worry about. He’s there to watch the referee mostly, as others have said, although if you’re new at that level he may sit near you to keep an eye on how you do. This is more for support than anything else. You will get useful feedback.
As Anubis says, if you’re not sure ask, but wait until he’s finished the briefing before doing so. I had one supply league observation where one of the NARs took over and kept butting in during the team talk. Then his phone went and he answered it! Teamwork went out the window and he struggled to finish the briefing. The game was challenging because he lost the thread and didn’t cover everything and then there was a mass confrontation.
Most importantly, don’t sweat it and enjoy the game.
